1. Field of the Invention
This invention generally relates to sintering of a valve metal powder for use as an anode in an electrolytic or an electrochemical/electrolytic hybrid capacitor. Suitable valve metals include, but are not limited to, tantalum, niobium, aluminum, titanium, and their alloys.
2. Prior Art
An electrolytic or an electrochemical/electrolytic hybrid capacitor can comprise a sintered porous anode body of valve metal such as tantalum or aluminum. Conventionally, the sintering process begins by molding a powder of the valve metal under pressure to form a molded piece or pellet having a desired shape. An anode lead wire of the valve metal is first imbedded into the powder before it is molded into the shaped pellet. The wire can also be welded to the pellet. The molded pellet with extending lead wire is then sintered in a furnace under a vacuum ranging from about 10−5 to about 10−6 Torr. Conventional sintering temperatures range from about 1,500° C. to about 1,800° C., and generally for as long as 30 minutes.
The problem is that when the pellet of a pressed valve metal powder is subjected to these temperatures for extended periods of up to 30 minutes, the real surface area and porosity of the anode are reduced. This, in turn, reduces anode volumetric capacitance and energy density. It also detrimentally affects the pellet's permeability, which means that powder particles deeper in the pellet are not readily available for the production of a dielectric oxide film thereon. This adversely increases capacitor ESR and decreases volumetric capacitance and energy density.